The conventional metal pump casing with PFA liner is widely used for chemical handling. In structure design, the pump includes either a stationary shaft or a rotary shaft. The supporting method of the stationary shaft includes a double-side-supporting structure or a cantilever supporting structure. In the double-side-supporting structure or the cantilever supporting structure, the triangle front support located in the suction channel and the rear shaft support at the containment shell, which are made of plastic, are used for supporting the front end and the rear end of the stationary shaft, respectively. The strength of the plastic is decreased when the operating temperature rises, and therefore the strength of the triangle front support and that of the blank rear support are decreased accordingly, which makes the stationary shaft crooked and displaced. Moreover, the metal pump casing with the PFA liner for large pumps has some problems about the manufacturing cost and yield. In the manufacturing process, the metal pump casing is secured in the mold, and then the PFA is introduced into the mold and combines with the metal pump casing into a single unit, wherein the metal pump casing usually includes a plurality of dovetail grooves so that the PFA liner can be firmly fixed onto the metal pump casing by the dovetail grooves.
There are three conventional ways, which are the transfer molding, the rotolining and the injection molding, to form the PFA liner on the inner surface of the metal pump casing. In the transfer molding process, the plastic (PFA) is loaded into a preheating chamber, heated, introduced from the preheating chamber through channels into the mold, and then cooled to a solid structure in the mold. The manufacturing cycle of the transfer molding process takes 8 to 12 hours so that the factories prepare several sets of the mold to increase the output per day. However, the high manufacturing cost of the transfer molding due to the long manufacturing time per manufacturing cycle cannot be overcome yet. In the rotolining process, the PFA powder is spread on the inner surface of the closed metal casing. However, without using a mold, the thickness of the liner cannot be precisely controlled, and the lower density of the liner allows higher permeability. Moreover, because the liner on the inner surface of the metal casing is formed by centrifugal force, the front support, located in the center of the pump, cannot be integrally formed with the liner of the casing. Moreover, the liner cannot be formed on the asymmetric volute channel of a centrifugal pump. In the injection molding process, the manufacturing speed is higher than the other ways of forming the liner, and a manufacturing cycle can normally be done in 10 minutes. However, it is improper to manufacture the liner of the large pump casing by the injection molding process, more particular to the liner of the large pump case with a suction size larger than 80 mm or an impeller size larger than 200 mm. Moreover, the liner of the centrifugal pump, in which the direction of the suction channel and the direction of the discharge channel are perpendicular to each other, will result in residual stress due to the interaction of the axial shrinkage stress and the radial shrinkage stress during the molding process. Such residual stress causes the liner to crack and resulting in pump failure. In addition, the residual stress in the liner can be released under high temperature and/or in a highly corrosive environment, and then cracks are generated in the liner. Therefore, the pump with the liner manufactured by injection molding is not suitable for transporting the chemical material at a temperature close to 200° C.
The following prior arts further describe the problems and the potential problems about the shaft supporting structure in the metal pump casing with the PFA liner and the manufacturing of the metal pump casing with the PFA liner.
Document 1
Document 1 is “The secret is in the lining: the use of fluoropolymer materials for corrosive pumping”, which is published in October, 2001 at WORLD PUMPS. Document 1, which is about manufacturing problems of the metal pump casing with the PFA liner, points out that manufacturing the liner by transfer molding requires heating the material of the liner and cooling the liner slowly. In addition, document 1 also points out that FTFE material is not suite for manufacturing the liner by transfer molding. The document 1 also points out that manufacturing the liner by rotolining has some difficulties in controlling thickness, density and flatness of the liner because this process does not take place under pressure and is done without a mold. Moreover, the document 1 points out that manufacturing the liner by injection molding is not suitable for manufacturing the large pump casing due to the shrinkage and the residual stress during the molding process.
Document 2
Document 2 is the catalog of the 3298 series product (www.itt.com) published by the American company, ITT Goulds Pumps, in 2014. Document 2, which is about the metal pump casing with the ETFE liner, points out that the ETFE liner is manufactured by rotolining. The figures in the catalog show that the casing includes a suction flange, a suction channel, a casing channel and a discharge flange which are integrally formed. The casing further includes the shaft support which is separately installed in the suction channel. In addition, as shown in the figures of the guide book, the casing channel in the casing does not show the feature that an area variation of the volute in the casing of the centrifugal pump.
Document 3
Document 3 is the guide book of the U-mag series product (www.innomag.com) published by the American company, INNOMAG, in 2010. Document 3, which is about the metal pump casing with the ETFE or PFA liner, points out that the ETFE liner is manufactured by rotolining. The shaft support is on the rear shaft seat, as a cantilever supporting structure. As shown in the figures of the guide book, the casing channel of the casing does not show the feature that an area variation of the volute in the casing of the centrifugal pump, and the casing of the pump does not have the shaft support.
Case 1
Case 1 is the U.S. Pat. No. 4,722,664, Lined corrosion resistant pump, issued in 1988. Case 1 points out that pump casing with the liner made of fluorocarbon polymer is suitable for transporting corrosive liquids. The material property of PFA is similar to PTFE, and can be processed by conventional melt processing techniques. The pump components made by PFA can be operated at temperatures higher than 150° C. However, the liner made of fluorocarbon polymer has two stress sources which are the residual stress generated during the molding process and the coefficient of thermal expansion difference between the PFA liner and the metal casing, wherein the PFA liner has a higher thermal expansion coefficient. In this case, a solution to the cracks generated by the shrinkage of the liner after molding is provided. The solution is that the fluorocarbon polymer is embedded in grooves and meshes to control the shrinking of the liner which generates cracks in the liner. However, the feature in this case is not used in the pump casing with the PFA liner manufactured by the applicant, the Duriron company. As a result, it is obvious that the grooves and the mesh used in this case for fixing the liner is too expensive and the result is similar to the conventional method in which the liner is fixed by dovetail grooves, and therefore the grooves and the mesh used in this case for fixing is not commercialized. In addition, whether injection molding can be used in manufacturing the liner of the large pump casing with a suction size larger than 80 millimeters (mm) or an impeller size larger than 200 mm is not mentioned in this case. The solution for the problem about the residual stress in the liner which is generated by injection molding is not provided in this case, too.
Case 2
Case 2 is the China patent NO. 2482597, Magnetic drive corrosion resistant fluorine plastic liner pump, issued in 2002. Case 2, which is about the metal pump casing with the PFA liner of the magnetic drive pump, discloses the structure of the PFA liner and that the pump casing with the PFA liner is suitable for transporting the corrosive liquid. The shaft support and the suction flange are integrally formed, and the shaft support as well as the suction flange can be detached from the pump casing. Although the shaft support and the suction flange are separated workpieces, the suction channel is not included in the detachable workpiece. Therefore, the area of the inner surface of the pump casing covered by the liner is still large, and the tensile stress in the liner covering the area between the suction channel and the volute channel is not eliminated. Moreover, the stiffness of the shaft support is not strengthened by reinforced material, and the reliability of the pump structure operated at 200° C. is not disclosed in this case.
Case 3
Case 3 is the U.S. Pat. No. 5,895,203, Centrifugal pump having separable, multipartite impeller assembly, issued in 1999. Case 3, which is about the metal pump casing with the plastic liner of the magnetic drive pump, discloses that the triangle front shaft support is a detachable workpiece, but the pump casing still includes the suction flange, the suction channel, the casing channel and the discharge flange. The connecting surface of the shaft support and the pump casing is covered by plastic without any metal having high stiffness or reinforced material connected directly in this case. Moreover, the reliability of the pump structure operated at 200° C. is not disclosed in this case.
Case 4
Case 4 is the EU patent NO. 2589811, Magnetic drive pump, issued in 2013. In Case 4, the inventor discloses a structural improvement of the shaft support in the pump casing with the PFA liner for operating at high temperatures. The stiffness of the plastic is decreased when the operating temperature rises, as well as the stiffness of the plastic shaft support is decreased accordingly, and, therefore, the metal shaft support with the PFA liner which is integrally formed with the pump casing with the PFA liner is provided to fulfill the high stiffness requirement of the shaft support operating at high temperature. Although the metal shaft support in this case solves the problem that the stiffness of the plastic shaft support is decreased at high operating temperatures, the metal shaft support integrally formed with the pump casing needs to be covered by the PFA liner encapsulated completely to isolate the pump casing, avoiding corrosion on the metal shaft support and the metal pump casing by corrosive chemical liquid. Moreover, the direction of the suction channel and the direction of the volute channel or the discharge channel are perpendicular to each other so that the residual stress, which is generated by the interaction of the axial shrinkage tension and the radial shrinkage tension, which come from the shrinkage during the molding process, is concentrated on the area of the thrust ring seat. This phenomenon is especially apparent in larger size pump casings. There is also the shrinkage stress left in the liner covering the shaft support which causes more serious stress concentration on the area of the thrust ring seat. In this case, for small pump casings, there is no crack appearing on the stress concentrated area of the liner after molding and proper heat treatment, however the crack appeared on the liner of the large pump casings which decreases the yield strength of the pump casing liner, especially for pump sizes larger than 3 inches×2 inches×8 inches (80 mm×50 mm×200 mm). In the large pump casing, the area ratio, the liner area on the volute channel compared to the total liner area on the pump casing, is greater. So, the residual stress, generated by the interaction of the axial shrinkage tension and the radial shrinkage tension, is significantly increased and concentrated at the thrust ring seat of the suction area. Therefore the yield strength of the liner covering the inner surface of the large pump casing is decreased even though proper heat treatment is applied on the liner after molding.
Please refer to FIG. 1A, in the prior art, the pump casing 4 is a single workpiece and are usually constructed of stainless steel or cast iron with a PFA liner. The PFA liner covers the inner surface of the pump casing 4. The pump casing 4 includes a suction flange 411, a suction channel 412, a shaft support 413, a volute channel 423, a discharge channel 422, a discharge flange 421 and a volute PFA liner 424. A plurality of dovetail grooves 47 are located on the inner surface of the suction channel 412 and the inner surface of the volute channel 423 for fixing the PFA liner 424. The volute PFA liner 424 includes a suction flange raised face 414a, a suction channel liner 414b, a shaft support liner 414c, a discharge flange raised face 424a, a discharge channel liner 424b and a volute channel liner 424c which are all integrally formed to isolate the metal parts of the pump casing 4 from corrosive liquids. A thrust ring 46 is installed on a thrust ring seat 461 located on the inner surface of the pump casing 4 so as to resist an axial thrust force generated by an impeller (not shown) when the pump is running.
If the PFA liner is manufactured by injection molding, the shrinkage of the liner covering the inner surface of the pump casing is restrained by the complex structure of the pump casing during the molding process. Particularly, the PFA liner covering the thrust ring seat 461 is pulled by an axial shrinkage stress Fa, a radial shrinkage stress Fb and a shrinkage stress Fc, and cracks are generated on the PFA liner covering the thrust ring seat 461. Therefore, the yield of the large pump casing with PFA liner is decreased in this case due to the large residual stress in the PFA liner.
Please refer to FIG. 1B, if the volute PFA liner of the pump casing is manufactured by transfer molding, the cycle of the transfer molding process takes 8 to 12 hours. Therefore, manufacturing the pump casing with PFA liner by transfer molding is uneconomic. If the PFA liner is manufactured by rotolining, the shaft support 414c and the volute channel liner 424c having the gradually increasing channel area is hard to be formed in right thickness, because the PFA powder covering the inner surface of the shaft support 414c and the volute channel liner 424c is driven by centrifugal force. Especially, the volute tongue 424d cannot be manufactured by rotolining, and the thickness as well as the flatness of the volute PFA liner 424 is difficult to control.
According to the above-mentioned documents, cases and figures, manufacturing the PFA liner of the metal pump casing has the following problems.
Problem 1
Manufacturing the PFA liner of the metal pump casing by transfer molding has a high manufacturing cost and takes a long time per manufacturing cycle.
Problem 2
Manufacturing the PFA liner of the metal pump casing by rotolining is not able to form the shape of volute casing and the surface of the shaft support which is integrally formed with the pump casing.
Problem 3
The PFA liner covering the inner surface of the metal pump casing manufactured by rotolining has low density, and the thickness of the PFA liner is hard to be controlled precisely.
Problem 4
Injection molding is not suitable for manufacturing the PFA liner of the large pump casing because the residual stress left in the PFA liner.
Problem 5
Injection molding is not suitable for manufacturing the PFA liner of the metal pump casing with the shaft support integrally formed with the metal pump casing because the residual stress left in the PFA liner is increased, so that injection molding is also not suitable for manufacturing the PFA liner of the large metal pump casing.
In manufacturing of the PFA liner of the metal pump casing, transfer molding has high manufacturing cost and takes long time per manufacturing cycle; rotolining is not able to form the PFA liner on the inner surface of the volute casing and the surface of the shaft support which is integrally formed with the pump casing, and the thickness of the PFA liner is hard to be controlled precisely. To solve the problems about manufacturing the PFA liner by injection molding such as the problem 4 and the problem 5 mentioned above are provided in the following.